He lost lung capacity while smoking, he quit so he's not anymore. On top of that he is actually winning that previously lost lung capacity back, the respiratory system is extremely good at "fixing" itself:
Initial lung capacity = I
Lost lung capacity = L
Current lung capacity = I - L
While (not smoking && Current lung capacity < Initial lung capacity) {
Current lung capacity++;
}
A rather vulgar formula from my aunt (a doctor): if you quit for (years you smoked * 3) you will have relatively clean lungs again.
Oh and people start smoking for the same reasons people start drinking: peer pressure, tradition, anti-conformism, teenage rebellion, to look cool, boredom, curiosity, etc...
...and then it becomes a fucking prison, extremely quickly.